Originally Posted by Bushmaster

285/70/R17 First number is tyre width as above: 275mm The second number is the side wall height as a percentage of the tyre width. As above 70% of the tyre width which would be: 192.5mm high. The third number is the size rim/wheel/mag you mount your tyre on. Again as above R17 is a standard Rubi 17" rim. R18 would be an 18" rim and so on and so forth. LT stands for light truck. There are heaps of other conventions but these are the easiest to explain. A tyre 285/70/R17 should measure 33" it will probably also star something about being mounted on a certain rim width only as well. I hope this helps.

Originally Posted by MD GP

Wider tires usually have higher rolling resistance. This will decrease mileage. Wider tires with the same aspect ratio will be taller and therefore greater in circumference. A taller tire would cause the speedometer, odometer and computer calculated mileage to be falsely low if that information is derived solely from rolling tires. I have a question though. I have noticed that my speedometer remains exactly correct compared to my in dash GPS, even though my tires have worn (and therefore become shorter). Do nav equipped jeeps get their speed and odometer readings from the GPS? Regarding the Procal, I'm sure it is an excellent unit. You might also consider the Superchips Flashcal. I like it because it does the same basic modifications, but uses a LCD and buttons instead of dip switches. It's also internet upgradable and the cost is similar. Even better, Superchips now produces a Flashpaq tuner for the 2012-13 Wranglers. It includes all of the Flashcal features and adds engine tuning capabilities. I have a question in to them now regarding a possible discounted upgrade path. Best regards, Dave

Originally Posted by TH3APPL3GUY

The in dash GPS speedo is linked to the normal speedometer, if you don't reprogram it will still show the incorrect reading